Acoustic technology is increasingly being used in the forestry and processing industries as a means of determining the inherent characteristics of wood and wood composite materials. It is a requirement of the building industry that the strength of a timber piece be sufficient for its purpose.
Utilising longitudinal acoustic signals as a probing means to measure a log's modulus of elasticity provides a convenient measure of stiffness or strength of a wood sample for the forestry industry, as such a measure is largely independent of the cross sectional area of the timber piece. Typically a stress wave is induced in the sample, be it a tree stem, log, or other wood piece, for example, by hitting it with a hammer. The modulus of elasticity can be derived from a measurement of the velocity of the stress wave in the sample length.
One type of instrument measures the time taken for a single traverse of the sample length (transit time) and, knowing the sample length, the acoustic velocity is calculated. This method necessitates transducing both ends of the sample, or alternatively one end of the sample and the hammer.
Most instruments use accelerometers to transduce the disturbance although in some instances displacement transducers are used. Commonly the stress wave is induced directly with a mechanical or pneumatic hammer, however the stress wave may also be induced by an electronic hammer e.g. Silvatest.
Usually the electronic hammer comprises an electronic method of exciting a piezoelectric transmitter or transducer. The controlling electronic signal may be used to indicate the excitation of the sample. The crux is that the stress wave transit time measure is the time measured between excitation of the stress wave and its detection at the receiving transducer.
A limitation to the usefulness of transit timer instruments is that the measure is prone to corruption by noise, due at least in part to the need for wide bandwidths to correctly identify starting and stopping points, and also corruption by other acoustic signals within the sample.
Another type of instrument records the reverberation of the stress wave within the sample for a duration equivalent to many transit periods. A single receiver transducer only is required.
The hit may occur at the same end as the receiving transducer. The hit must contain frequencies which match and excite the resonances of the sample hence ideally an impulse is required which has fast transitions and short period.
The spectral composition of the reverberation is determined typically by Fourier analysis and, knowing the sample length, the velocity calculated.
Since many transits of the sample are recorded the calculated velocity is an average for the recording duration, preferably dominated by the plane wave reverberation.
To accurately determine a sample's velocity it is a requirement that the combination of hit amplitude and material absorption be such that the resonance is recorded for many reverberations. The sample's acoustic absorption dampens the stress wave and imparts an effective window function on the spectral signature. As the absorption increases the resonance peaks broaden resulting in reduced accuracy. Generally resonance is less susceptible to random noise; interference on the other hand appears in the output masquerading as a sample resonance.
In both of the previous instrument types the measured response is of stress waves which have travelled the entire length of the stem. The acoustic velocity calculated from such a response is therefore an average of the entire stem. Those skilled in the art know that the velocity varies along the stem, depending on factors such as, but not limited to, variations in density and stiffness, and the presence of knot whirls and other large discontinuities in the structure.
Therefore a characteristic of the stem determined from the average velocity may differ significantly from the value of the characteristic for a portion of the stem. The average velocity of the stem may be useful in grading whole trees, but cannot be used to determine such things as the location of cuts along the stem in order to maximise the value of the logs. This can only be done using a velocity calculated for the part of the stem about to be cut.
An adaptation on these instrument types, sometimes called a standing tree tool, is to measure a short section of the sample length. Two accelerometers are hit into the log or tree about one metre apart. A stress wave is induced in the log or tree, typically a hit, and a measurement undertaken.
This adaptation suffers from interpretation difficulties relating to the spread of the acoustic signal from the launch point into the bulk of the log or tree and along the surface to the second accelerometer. The pulse may not spread spherically due to the radial velocity profiles within the log or tree, and will not travel as a plane wave.
This adaptation is known to provide a velocity that is also dependant on the diameter of the log. PCT publication WO 02/060662 “System for and method of performing evaluation techniques on a log or round timber” addresses a diameter correction to this adaptation.
Only a small section of a log, in most embodiments about one metre, is measured and it is assumed that this is representative of the tree. However the average values and radial profiles of trees vary notably within the tree, reducing the usefulness of this adaptation.
In forestry applications measurements using the above instruments are either carried out prior to the tree being felled, as in the standing tree tool, or after harvesting when the log has already been cut from the stem.
The instruments described above generally require manual operation. In forestry applications this means provision of safe access to the tree for the operators of the instruments. Access to the stem during the harvesting process, when the information may be of greatest use, is unlikely to be possible or practicable for safety reasons.
Modern harvesting methods increasingly use mechanical harvesters. These machines cut the tree stem from the stump, remove side limbs and in some cases strip bark from the stem prior to cutting the stem into logs of typically around 6 m in length.
It is unsafe to make manual measurements of the stem during this harvesting process, and it is not economical to stop the harvester while the measurement is taken. Therefore the log is generally cut before the physical characteristics (eg stiffness) of the log are determined, thus losing the opportunity to adjust the cut log length to optimize value.
New Zealand Patent No 505896 “Method and Apparatus for assessing or predicting characteristics of wood and other materials” provides an alternative reverberation method. This patent gives an example of a possible harvester head implementation for optimizing log cutting using acoustics to determine an average wood velocity for the complete stem. In this instance the acoustic wave is introduced into the stem by using the harvester head to shake the stem, and reverberation analysis to determine the average velocity of the complete stem.
This method provides information regarding the whole stem rather than the section of the stem about to be cut. It therefore suffers from the same disadvantages as the other “whole of log” methods discussed above. In particular, it cannot be used directly to determine the cutting position and hence to optimize the value of any specific log.
Patents exist considering the detection and grading of logs for defects within a harvester head, for example U.S. Pat. No. 5,097,881 “Ultrasonic log grading”. This patent ultrasonically detects internal defects such as knots by analysing the radial response (ie of waves travelling across the stem). The method disclosed does not measure longitudinal sonic velocity as a measure of wood quality and therefore does not provide a measure of (longitudinal) stiffness.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.